Fix a Javadoc typo that crashes Checkstyle (yes, really)

Change-Id: Ib0ca5ad40a8e68a21c707941c7d533c9206a9129

server/src/main/java/com/vaadin/server/JsonPaintTarget.java

@@ -158,7 +158,7 @@ public class JsonPaintTarget implements PaintTarget {
      * 
      * @param tag
      *            the name of the end tag.
-     * @throws Paintexception
+     * @throws PaintException
      *             if the paint operation failed.
      */